US 8088324 B2
Apparatus and method for producing fibrous materials in which the apparatus includes an enclosure having an inlet configured to receive a substance from which the fibrous materials are to be composed, a common electrode disposed in the enclosure, and plural extrusion elements provided in a wall of the enclosure opposite the common electrode so as to define between the plural extrusion elements and the common electrode a space in communication with the inlet to receive the substance in the space. In the method, a substance from which the fibrous materials are to be composed is fed to the enclosure having the plural extrusion elements, a common electric field is applied to the extrusion elements in a direction in which the substance is to be extruded, the substance is extruded through the extrusion elements to tips of the extrusion elements, and the substance is electrosprayed from the tips to form the fibrous materials.
1. A method for producing fibrous materials, comprising:
feeding a substance from which the fibrous materials are to be composed to an enclosure having at least one extrusion element, said at least one extrusion element including at least one frit segment entirely covering an opening in the extrusion element;
applying a common electric field to said extrusion element in a direction in which said substance is to be extruded;
extruding said substance through said extrusion element including said at least one frit segment; and
electrospraying said substance at the tips of said plural extrusion elements to form said fibrous materials.
2. The method of
electrospinning said extruded substance from said at least one extrusion element to form fibers.
3. The method of
electrospinning said extruded substance from said at least one extrusion element to form nanofibers.
4. The method of
electrospinning said fibrous materials in said common electric field having a strength of 2,000-400,000 V/m.
5. The method of
collecting said fibrous materials on a collector.
6. The method of
collecting fibers of said extruded substance.
7. The method of
electrospinning polymeric fibers.
8. The method of
annealing said polymeric fibers to form carbon fibers.
9. The method of
electrospinning polymeric nanofibers.
10. The method of
annealing said polymeric nanofibers to form carbon nanofibers.
This application is a Divisional of application Ser. No. 10/819,942, filed Apr. 8, 2004 now U.S. Pat. No 7,762,801, entitled “Electrospray/Electrospinning Apparatus and Method”, which is incorporated herein by reference. application Ser. No. 10/819,942 is related to U.S. application Ser. No. 10/819,916, filed on Apr. 8, 2004, entitled “Electrospinning of Fibers Using a Rotating Spray Head”, the entire contents of which are incorporated herein by reference. This application is related to U.S. application Ser. No. 10/819,945, filed on Apr. 8, 2004, entitled “Electrospinning in a Controlled Gaseous Environment”, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to the field of electrospraying and electrospinning of fibers or fibrous materials from polymer solutions.
2. Background of the Invention
Nanofibers are useful in a variety of fields from clothing industry to military applications. For example, in the biomaterial field, there is a strong interest in developing structures based on nanofibers that provide a scaffolding for tissue growth effectively supporting living cells. In the textile field, there is a strong interest in nanofibers because the nanofibers have a high surface area per unit mass that provides light but highly wear-resistant garments. As a class, carbon nanofibers are being used for example in reinforced composites, in heat management, and in reinforcement of elastomers. Many potential applications for nanofibers are being developed as the ability to manufacture and control their chemical and physical properties improves.
Electrospray/electrospinning techniques are used to form particles and fibers as small as one nanometer in a principal direction. The phenomenon of electrospray involves the formation of a droplet of polymer melt at an end of a needle, the electric charging of that droplet, and an expulsion of parts of the droplet because of the repulsive electric force due to the electric charges. In electrospraying, a solvent present in the parts of the droplet evaporates and small particles are formed but not fibers. The electrospinning technique is similar to the electrospray technique. However, in electrospinning and during the expulsion, fibers are formed from the liquid as the parts are expelled.
Glass fibers have existed in the sub-micron range for some time. Small micron diameter electrospun nanofibers have been manufactured and used commercially for air filtration applications for more than twenty years. Polymeric melt blown fibers have more recently been produced with diameters less than a micron. Several value-added nonwoven applications, including filtration, barrier fabrics, wipes, personal care, medical and pharmaceutical applications may benefit from the interesting technical properties of commercially available nanofibers and nanofiber webs. Electrospun nanofibers have a dimension less than 1 μm in one direction and preferably a dimension less than 100 nm in this direction. Nanofiber webs have typically been applied onto various substrates selected to provide appropriate mechanical properties and to provide complementary functionality to the nanofiber web. In the case of nanofiber filter media, substrates have been selected for pleating, filter fabrication, durability in use, and filter cleaning.
A basic electrospinning apparatus 10 is shown in
The electrospinning process has been documented using a variety of polymers. One process of forming nanofibers is described for example in Structure Formation in Polymeric Fibers, by D. Salem, Hanser Publishers, 2001, the entire contents of which are incorporated herein by reference. By choosing a suitable polymer and solvent system, nanofibers with diameters less than 1 micron can be made.
Examples of fluids suitable for electrospraying and electrospinning include molten pitch, polymer solutions, polymer melts, polymers that are precursors to ceramics, and/or molten glassy materials. These polymers can include nylon, fluoropolymers, polyolefins, polyimides, polyesters, and other engineering polymers or textile forming polymers. A variety of fluids or materials besides those listed above have been used to make fibers including pure liquids, solutions of fibers, mixtures with small particles and biological polymers. A review and a list of the materials used to make fibers are described in U.S. Patent Application Publications US 2002/0090725 A1 and US 2002/0100725 A1, and in Huang et al., Composites Science and Technology, v63, 2003, the entire contents of which are incorporated herein by reference. U.S. Patent Application Publication No. US 2002/0090725 A1 describes biological materials and bio-compatible materials to be electroprocessed, as well as solvents that can be used for these materials. U.S. Patent Application Publication No. US 2002/0100725 A1 describes, besides the solvents and materials used for nanofibers, the difficulties of large scale production of the nanofibers including the volatilization of solvents in small spaces. Huang et al. give a partial list of materials/solvents that can be used to produce the nanofibers.
Further, U.S. Pat. No. 3,280,229, the entire contents of which are incorporated herein by reference, describes metal needles for electrospinning via single or multiple electrified needles. Alternatively, electrospinning can occur from a receptor having a narrow end through which the fluid can exit the receptor and a long pointed electrode immersed in the fluid to electrify the fluid. For example, U.S. Pat. No. 705,691, the entire contents of which are incorporated herein by reference, describes a simple spray head as described above.
Further, U.S. Patent Application Publication Nos. US 2002/0007869A1, US 2002/0090725A1, US 2002/0100725A1, US 2002/0122840A1, and US 2002/0175449A1, the entire contents of which are incorporated herein by reference, describe a plurality of electrified needles used to increase a spray area for nanofiber production. These patent applications disclose methods by which a polymer fiber is distributed to a plurality of needles, each needle being connected to one or more conductive boards that have a high voltage. For example, U.S. Patent Application Publication No. US 2002/0122840A1 shows an apparatus for electrospinning in FIG. 2 a in which two conductor boards 26 and 30 make electrical contact to each needle 32. A high voltage is applied to each needle 32 through the conductor boards 26 and 30 that are in direct contact with the needles. Further, both U.S. Patent Publication Appl. No. 2002/0122840A1 and U.S. Pat. Publication Appl. No. US2002/0175449A1, describe electrospinning of polymer solutions through one or more charged conducting nozzles arranged on at least one conducting plate.
Hence, the background techniques using a multiplicity of individually electrified needles and/or a multiplicity of solution reservoirs are not conducive to large scale manufacturing. The number of controls necessary to control the electrical field at each needle scales with the number of needles, which may easily exceeds 100 needles for large scale production. Further, the control and delivery of the polymer solutions separately to each needle reservoir complicate the scale up to large scale nanofiber production.
One object of the present invention is to provide an apparatus and a method for the production of fibers and/or fibrous materials conducive to mass production.
Another object is to provide an apparatus and a method which produce fibers and/or fibrous materials in a parallel production process that ameliorate the deficiencies of the background art discussed above.
Accordingly, a further object of the present invention is to provide an apparatus and a method which simultaneously extrudes a plurality of fibers and/or fibrous materials from an electrospray head.
Thus, according to one aspect of the present invention, there is provided a novel apparatus for producing fibrous materials, including an enclosure having an inlet configured to receive a substance from which the fibrous materials are to be composed, a common electrode disposed in the enclosure, and plural extrusion elements provided in a wall of the enclosure opposite the common electrode so as to define between the plural extrusion elements and the common electrode a space in communication with the inlet to receive the substance in the space.
According to a second aspect of the present invention, there is provided a novel method that feeds a substance from which the fibers are to be composed to the enclosure having the plural extrusion elements, applies a common electric field to the extrusion elements in a direction in which the substance is to be extruded, extrudes the substance through the plural extrusion elements to tips of the extrusion elements, and electrosprays the substance from the tips to form the fibrous materials.
extrudes the substance through the extrusion elements in the common electric field.
A more complete appreciation of the present invention and many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Referring now to the drawings, wherein like reference numerals designate identical, or corresponding parts throughout the several views, and more particularly to
The electrode 26 in one embodiment of the present invention is centered within the enclosure and forms a common electrode producing a common electric field for extruding the electrospray medium. Preferably, the electrode 26 can be disposed close to but not in contact with the extrusion elements 30. An exterior electrode 35 is provided outside the enclosure 28 facing the electrode 26. An electric potential across to the electrodes 26 and 35 establishes an electric field 12 as shown in
In one embodiment of the present invention, the spray head 24 preferably includes individual extrusion elements 30 such as for example capillaries, bundles of capillaries, needles, bundles of needles, tubes, bundles of tubes, rods, bundles of rods, concentric tubes, frits, open-cell foams, combinations thereof, or otherwise channels of appropriate shape formed in a wall of the enclosure 28. The individual extrusion elements can be made of metal, glass, or plastic capillary tubes appropriately sized to deliver the electrospray medium 14 from the spray head 24 to an exterior of the spray head 24, where the electrospray medium 14 is electrified. Further, the extrusion elements 30, in one embodiment of the present invention, as shown in
Generally, smaller diameter tubes yield a narrower nanofiber. Also, while multiple tubes (spray heads) can be accommodated in a single device, a certain minimum distance must be allowed between the adjacent tubes to avoid electrical interference between them. The minimum distance varies with one or more of the polymer/solvent system used, the electric field density, and the tube diameter. Tubes placed too close to each other can cause slower solvent removal rates affecting the fiber quality.
The extrusion elements 30, in one embodiment of the present invention, are arrayed in channels placed adjacent or close to each other in one or more directions. These channels can be bundles of individual members in the form, for example, capillaries or rods close to each other. The individual members can be made of, for example, non-conducting materials such as glass, ceramic, Teflon, or polyethylene but also of conducting materials. The use of a multiplicity of electrically insulating extrusion elements 30 made of electrically insulating or non-conducting materials does not alter the electric field 12 established between the electrode 26 and the exterior electrode 35.
In another embodiment shown in
The use of the electrode 26 in a configuration with multiple extrusion elements 30 permits a high throughput without the complexity of selectivity controlling electric fields singularly at each extruding element. Further,
Further, according to another embodiment of the present invention,
The electrode 26 in the embodiment of
The high voltage source HV is connected to the electrode 26 through a lead 44 and to the exterior electrode 35 through another lead 46 as shown in
Typically, the exterior electrode 35 is grounded, and the fibers produced by extrusion from the extrusion elements 30 are directed by the electric field 12 toward the exterior electrode 35. Electrospun fibers or electrosprayed fibrous materials in one embodiment of the present invention can be collected by a collecting mechanism such as a conveyor belt 50 as schematically shown in
The distance between the exterior electrode 35 and the electrode 26 is determined based on a balance of a few factors such as for example a time for the solvent evaporation rate, the electric field strength, and a distance/time sufficient for a reduction of the fiber diameter. These factors and their determination are similar in the present invention to those in conventional single needle spray elements. The present inventors have discovered that a rapid evaporation of the solvents results in larger than nm-size fiber diameters.
Therefore, in one embodiment of the present invention, the evaporation of the solvent is controlled by placing the enclosure 28 in a chamber 52 as shown in
Control of the gaseous environment about the extrusion elements 30 improves the quality of the fibers electrospun with regards to the distribution of nanofiber diameter and with regards to producing smaller diameter nanofibers. The present inventors have discovered that the introduction into the gaseous environment about the extrusion elements of electronegative gases such as for example carbon dioxide, sulfur hexafluoride, and freons, and gas mixtures including vapor concentration of solvents, ions, and/or charged particles improves the quality of electrospun fibers (i.e., the fibers are smaller in diameter and have a closer distribution of diameter sizes).
While electronegative gases such as carbon dioxide have been utilized in electrospraying to generate particles and droplets of material, no effects prior to the present work have been shown for the utilization of electronegative gases in an electrospinning environment. Indeed, the nature of electrospinning in which liberal solvent evaporation occurs in the environment about the extrusion elements and especially at the liquid droplet at the tip of the extrusion element would suggest that the addition of electronegative gasses would not influence the properties of the spun fibers.
Further, the differences in fluid properties of the polymer solutions utilized in electrospraying and those utilized in electrospraying, such as for example differences in conductivity, viscosity and surface tension, result in quite different gaseous environments about electrospraying and electrospinning apparatuses. For example, in the electrospray process, a fluid jet is expelled from a capillary at high DC potential and immediately breaks into droplets. The droplets may shatter when the evaporation causes the force of the surface charge to exceed the force of the surface tension (Rayleigh limit). Electrosprayed droplets or droplet residues migrate to a collection (i.e., typically grounded) surface by electrostatic attraction. Meanwhile in electrospinning, the highly viscous fluid utilized is pulled (i.e., expelled) as a continuous unit as an intact jet because of the inter-fluid attraction, and is stretched as the pulled fiber dries and undergoes the instabilities described below. The drying and expulsion process reduces the fiber diameter by at least 1000 times. In electrospinning, the present invention recognizes that the complexities of the process are influenced by the gaseous atmospheres surrounding the pulled fiber, if polymer solutions with relatively low viscosities and solids content are to be used to make very fine fibers (i.e., less than 100 nm in diameter).
With reference to
By modifying the gaseous environment surrounding the capillary, the present invention permits increases in the applied voltage and improved pulling of the liquid jet from the capillary. In particular, electronegative gases appear to reduce the onset of a corona discharge around the capillary thus permitting operation at higher voltages enhancing the electrostatic force. The formation of corona around the capillary would disrupt the electrospinning process. Further, according to the present invention, insulating gases will reduce the possibility of bleed-off of charges in the Rayleigh instability region, thereby enhancing the stretching and drawing of the fiber. Cross-referenced related application U.S. application Ser. No. 10/819,945, entitled “Electrospinning in a Controlled Gaseous Environment,” contains further details of controlling and modifying the gaseous environment during electrospinning.
The drying rate for the electrospun fiber during the electrospining process can be adjusted by altering the partial pressure of the liquid vapor in the gas surrounding the fiber. Retarding the drying rate would be advantageous because the longer the residence time of the fiber in the region of instability the more prolonged is the stretching, and consequently the smaller the diameter of the resultant fiber. The height of the containment chamber and separation of the capillary at high DC voltage from the ground need, according to the present invention to be compatible with the drying rate of the fiber. Also the DC voltage is preferably adjusted to maintain an electric field gradient of about 3 KV/cm.
As illustrative of the electrospinning process of the present invention, the following non-limiting examples are given to illustrate selection of the polymer, solvent, extrusion element to collection surface separation, solvent pump rate, and addition of electronegative gases. One illustrative example for selection, according to the present invention, of polymer, solvent, extrusion element, collection surface separation, solvent pump rate, and addition of electronegative gases is given below:
a polymer solution of a molecular weight of 350 kg/mol,
a solvent of dimethylformamide DMF,
an extrusion element tip diameter of 1000 μm,
an Al plate collector,
˜0.5 ml/hr pump rate providing the polymer solution,
an electronegative gas flow of CO2 at 8 lpm,
an electric field strength of 2 KV/cm, and
a gap distance between the tip and the collector of 17.5 cm.
A decreased fiber size can be obtained by increasing the molecular weight of the polymer solution to 1000 kg/mol, and/or introducing a more electronegative gas (such as for example Freon), and/or increasing gas flowrate to for example 20 lpm, and/or decreasing the tip diameter to 150 μm (e.g. as with a Teflon tip).
Thus, the gaseous environment surrounding the extrusion elements during electrospinning influences the quality of the fibers produced. Indeed, the present inventors have observed that the electrospinning process can be started and stopped by turning on or off a supply of an electronegative gas. Blending gases with different electrical properties can be used to optimize performance. One example of a blended gas includes CO2 (at 4 lpm) blended with Argon (at 4 lpm).
Further, when a solvent such as methylene chloride or a blend of solvents is used to dissolve the polymer, the rate of evaporation of the solvent will depend on the vapor pressure gradient between the fiber and the surrounding gas. The rate of evaporation of the solvent can be controlled by altering the concentration of solvent vapor in the gas. The rate of evaporation affects the Rayleigh instability. In turn, the electrical properties of the solvent and its vapor influence the electrospinning process. For example, by maintaining a liquid solvent pool at the bottom of a chamber, the amount of solvent vapor present in the ambient about the electrospinning is controlled by altering the temperature of the chamber and/or pool, and thus controlling the partial pressure of the solvent in the gaseous ambient about the electrospinning. Having a solvent vapor in the electrospinning chamber affects the drying rate of the fibers, and alters the fiber surface characteristics when a solvent other than the one used in spinning solution is used in the chamber.
While the effect of controlling the environment about an electrospinning extrusion element has been illustrated by reference to
A non-planar electrode configuration is shown in
According to one embodiment of the present invention, the enclosure 28 can be made by micro-machining holes with an appropriate configuration in a flat or appropriate shaped plate of Al or silicon, which is subsequently oxidized to silicon dioxide. Lasers can be used according to the present invention to micro-machine the Al or silicon plate by selectively ablating nearly all the material within a focal spot of the laser beam before any significant heat conduction or mass flow takes place, thus enabling precise machining with little thermal damage. For example, using a Q-switched Nd: YAG and excimer lasers, a 60 fs laser with a 5 μm focused spot can produce holes as small 800 nm in SiO2, and 300 nm diameter in metal films. Other lasers and fabrication techniques known to one skilled in the art and including but not limited to chemical etching and electromechanical machining can be used for micro-machining the enclosure 28 and other parts of the present invention.
For the purposes of an exemplary teaching, the electrode 26 with a plurality of extrusion elements, as depicted in
In this exemplary teaching, the electrode 26 can be formed from a piece of metal by a machining or turning process (e.g. turning a metal disc to an outside diameter of 1.75 cm and then slicing the disk and machining the sliced disk to a prescribed thickness, such a for example 0.25 cm). The metal can be a soft or refractory metal. Lead connections can be soldered or welded to the electrode.
Having formed the electrode, the enclosure can be formed by the fabrication of two separate components. With reference to
Having now formed the first component 66 of the enclosure 28, the second component 72 (i.e., the wall of the enclosure 28 containing the extrusion elements 30) can be fabricated. Once again, an intrinsic Si wafer or a silica disc can be used. In either case, if the outside diameter is oversized, diamond turning or laser machining can be used to set the outside diameter for clearance of ID1 (i.e. under 1.8 cm). Laser drilling or lithography/etching can be used to form an array of openings in the second component 72 as shown in
Having the major pieces of the spray head 24 fabricated, the electrode 26 is inserted into the cavity 68 beyond the stop 70. Rubber stops 74 can be used to locate the electrode 26 above and below the stop 70 as shown in
Other materials besides those described above can be used to fabricate the spray head. For example, the present inventors have found that plastics and polytetrafluoroethylene can be used for the first component 66 of the enclosure and as well as the second component 72. Further, silicone rubber can be used as well for these components. If a rubber wall is used for the second component 72, then the rubber wall can be cut slightly larger than the opening of the first component 66 to frictionally fit the first component 66. Moreover, the extrusion elements 30 can be manufactured for example from commercially available glass tubes that are thinned to a desired inside dimension, cut into pieces and inserted into the rubber wall.
Thus, the present invention provides various apparatuses and methods for producing fibrous materials. As depicted in
The electrospraying can electrospin the extruded substance from the plural extrusion elements to form fibers or nanofibers. The electrospraying preferably occurs in an electric field strength of 2,000-400,000 V/m. The fibrous materials electrosprayed from the extrusion elements are collected on a collector. The fibers electrospun from the extrusion elements can also be collected on a collector. The fibers produced can be nanofibers.
The fibers and nanofibers produced by the present invention include, but are not limited to, acrylonitrile/butadiene copolymer, cellulose, cellulose acetate, chitosan, collagen, DNA, fibrinogen, fibronectin, nylon, poly(acrylic acid), poly(chloro styrene), poly(dimethyl siloxane), poly(ether imide), poly(ether sulfone), poly(ethyl acrylate), poly(ethyl vinyl acetate), poly(ethyl-co-vinyl acetate), poly(ethylene oxide), poly(ethylene terephthalate), poly(lactic acid-co-glycolic acid), poly(methacrylic acid) salt, poly(methyl methacrylate), poly(methyl styrene), poly(styrene sulfonic acid) salt, poly(styrene sulfonyl fluoride), poly(styrene-co-acrylonitrile), poly(styrene-co-butadiene), poly(styrene-co-divinyl benzene), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), poly(vinylidene fluoride), polyacrylamide, polyacrylonitrile, polyamide, polyaniline, polybenzimidazole, polycaprolactone, polycarbonate, polydimethylsiloxane-co-polyethyleneoxide, polyetheretherketone, polyethylene, polyethyleneimine, polyimide, polyisoprene, polylactide, polypropylene, polystyrene, polysulfone, polyurethane, polyvinylpyrrolidone, proteins, SEBS copolymer, silk, and styrene/isoprene copolymer.
Additionally, polymer blends can also be produced as long as the two or more polymers are soluble in a common solvent. A few examples would be: poly(vinylidene fluoride)-blend-poly(methyl methacrylate), polystyrene-blend-poly(vinylmethylether), poly(methyl methacrylate)-blend-poly(ethyleneoxide), poly(hydroxypropyl methacrylate)-blend poly(vinylpyrrolidone), poly(hydroxybutyrate)-blend-poly(ethylene oxide), protein blend-polyethyleneoxide, polylactide-blend-polyvinylpyrrolidone, polystyrene-blend-polyester, polyester-blend-poly(hyroxyethyl methacrylate), poly(ethylene oxide)-blend poly(methyl methacrylate), poly(hydroxystyrene)-blend-poly(ethylene oxide)).
By post treatment annealing, carbon fibers can be obtained from the electrospun polymer fibers.
Numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.